This invention relates generally to computed tomography (CT) imaging and more particularly to volumetric reconstruction of a cyclically moving object using digital area detector technology.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped x-ray beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system which is generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The array may be a linear detector array where individual detectors are aligned in a row or it may be an area detector where individual detectors form a two-dimensional array. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the x-ray intensity incident on the detector element, enabling computation of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce an attenuation profile.
In known “third generation” CT systems, the orientation of the x-ray source and the detector array are fixed and rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object changes. From the x-ray attenuation measurements, one computes the integral of the linear attenuation coefficient along a volume connecting the x-ray source with each detector element. This data is known as projection data, and when generated from the detector array at one gantry angular position, is referred to as a “view”. A “scan” of the object includes a collection of views made at different angular positions of the gantry relative to the object being scanned, or view angles, during one or more rotations of the x-ray source and detector about the object. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. Moreover, if the particular embodiment utilizes a two-dimensional detector, a volumetric reconstruction of the object being scanned may be generated. In this configuration, the scan data acquired from the object is not mathematically complete; however, the images may be useful for imaging evaluations.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back-projection technique. In some implementations, this process converts the collection of attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In current, state-of-the-art “third generation” CT systems, gantries have a rotational period of approximately 0.5 seconds. A rotational period of 0.5 seconds is sufficient to arrest or “freeze” most motion within the human body such as minor patient movement during scanning. However, this period is too long to arrest motion in axial images of the heart due to cardiac motion. As a result, novel reconstruction techniques have been developed to improve the temporal resolution in axial reconstructions of the heart, which facilitates a reduction in image artifacts caused by motion of the heart.
One such approach uses projection data acquired over an arc of the complete rotation of the gantry. This approach is called a segment reconstruction strategy and utilizes data acquired at view angles covering an arc length of 180 degrees plus the fan angle of the x-ray beam. For a system with a gantry period of 0.5 seconds, the temporal resolution in reconstructed axial images can be improved to approximately 330 milliseconds. To further improve the temporal resolution in reconstructed images, projection data acquired over multiple rotations of the gantry can be combined. For example, the temporal resolution can be improved to approximately 170 milliseconds if projection data at view angles covering an arc that is half of the arc utilized for the segment reconstruction approach is acquired during one rotation of the gantry, and the remaining projection data is acquired during a subsequent rotation. This approach is called a multi-sector reconstruction algorithm. The multi-sector reconstruction algorithm relies on appropriate selections of gantry speed and helical pitch, which match with the heart rate of the patient being imaged.
The predictability and regularity of the cyclically moving object during the scanning interval inherently limit the temporal resolution achievable with multi-sector reconstruction algorithms. This limitation restricts the ability to diagnose diseases of the coronary vasculature such as the development of atherosclerotic plaque deposits. To further improve the diagnostic potential from reconstructed images, the spatial resolution of existing CT technology can also be increased to facilitate accurate and reliable detection of stenoses in coronary vessels.